The use of thermal energy to destroy bodily tissue can be applied to a variety of therapeutic procedures, including the destruction of tumors. Thermal energy can be imparted to the tissue using various forms of energy, such as radio frequency electrical energy, microwave or light wave electromagnetic energy, or ultrasonic vibrational energy. Radio frequency (RF) ablation, for example, can be effected by placing one or more electrodes against or into tissue to be treated and passing high frequency electrical current into the tissue. The current can flow between closely spaced emitting electrodes or between an emitting electrode and a larger, common electrode located remotely from the tissue to be heated.
One disadvantage with these techniques is that maximum heating often occurs at or near the interface between the therapeutic tool and the tissue. In RF ablation, for example, the maximum heating can occur in the tissue immediately adjacent to the emitting electrode. This can reduce the conductivity of the tissue, and in some cases, can cause water within the tissue to boil and become water vapor. As this process continues, the impedance of the tissue can increase and prevent current from entering into the surrounding tissue. Thus, conventional RF instruments are limited in the volume of tissue that can be treated.
Fluid enhanced ablation therapy, such as the SERF ablation technique (Saline Enhanced Radio Frequency™ ablation), can treat a greater volume of tissue than conventional RF ablation. The SERF ablation technique is described in U.S. Pat. No. 6,328,735, which is hereby incorporated by reference. Using the SERF ablation technique, saline is passed through a needle and heated, and the heated fluid is delivered to the tissue immediately surrounding the needle. The saline helps distribute the heat developed adjacent to the needle and thereby allows a greater volume of tissue to be treated with a therapeutic dose of ablative energy. The therapy is usually completed once a target volume of tissue reaches a desired therapeutic temperature, or otherwise receives a therapeutic dose of energy.
Fluid enhanced ablation therapy generally creates a spherically-shaped treatment zone in tissue surrounding an ablation device. In some situations, however, it can be desirable to create a treatment zone having a non-spherical shape. For example, some lesions or tumors suitable for treatment with fluid enhanced ablation are not spherical in shape. In addition, it can be desirable to protect certain structures, such as sensitive nerve cells, located very near to a lesion or other target volume of tissue.
In still other situations, it can be desirable to introduce directionality into a treatment zone created using ablation therapy. For example, a common treatment for cardiac dysrhythmias, such as atrial fibrillation, involves a catheter-based procedure that selectively ablates tissue in the atrial walls in order to create defined pathways for the passage of the electrical signals that drive heartbeats. Current methods of ablation therapy, however, cannot create treatment zones similar to these directional pathways in the ventricle because they cannot heat through the ventricle wall, so this technique cannot be applied to the treatment of ventricular tachycardia. Fluid enhanced ablation is capable of heating through the ventricle wall, but the ablation zones created are so large that it is not possible to create pathways for the electrical signals, as too much of the heart is ablated.
Accordingly, there remains a need for improved devices and methods for shaping a treatment zone created during fluid enhanced ablation therapy.